1. Field of the Invention
The present invention relates to a waveguide optical element in which a thin film such as an antireflection film or a high-reflectance film is formed at an end face of a waveguide and its driving method and, more particularly, to a waveguide optical element used as an optical amplifier in, e.g., an optical communication system, and its driving method.
2. Related Background Art
A conventional travelling wave type laser amplifier is formed in such a manner that a semiconductor active layer serving as a waveguide is formed on a substrate, and an electrode for injecting a current to the active layer is arranged. In this amplifier, the two end faces of the active layer are cleavaged to constitute a Fabry-Perot resonator.
When the amplifier is to be driven, a current equal to or lower than a threshold value, i.e., a current value for causing the amplifier itself to emit light is injected from the electrode to the active layer. Light is caused to be incident from one end face. The incident light is amplified by induced emission in the active layer, and emerges from the other end face.
In order to increase a gain of such a laser amplifier, it is known to form an antireflection film on the end face of the resonator. When an antireflection film is formed in this manner, the above-mentioned threshold value current is increased, and the density of carriers capable of being injected into the active layer can be increased. When the carrier density in the active layer is increased, a gain of light incident on the amplifier can be increased accordingly. The Fabry-Perot resonator has very severe wavelength selectivity in its light transmission characteristics. The antireflection film also has an effect of reducing the wavelength selectivity, and widening a wavelength range of light which can be used for the amplifier.
However, in the conventional amplifier, the refractive index, the film thickness, and the like of the antireflection film are determined to obtain a maximum antireflection effect against light having a predetermined wavelength at a predetermined temperature. For this reason, the conventional amplifier cannot obtain a sufficient gain when the wavelength of light to be actually used is different from the predetermined wavelength, or when a temperature during an operation is changed. The same problems are posed not only in the amplifier described above, but also in a laser diode in which a high-reflectance film is formed on an end face of a resonator.